1. Field of the Invention
The present invention relates to an image forming apparatus and a development device, and more specifically to a mechanism for stirring a developer.
2. Discussion of the Background Art
In an image forming apparatus such as a copier, a printer, and a facsimile machine, an electrostatic latent image formed on a photoconductor constituting a latent image carrying member is developed by a development device and transferred onto a sheet or the like, to produce a record output.
The image forming apparatus may be configured to include a single photoconductor for a single color or a plurality of photoconductors for forming images of a plurality of colors. The latter configuration is used to form a multicolor image, including a full-color image.
Types of developer used in development include a one-component developer, containing only magnetic or nonmagnetic toner, and a two-component developer containing a mixture of toner and carrier. The two-component developer contains toner and carrier for carrying the toner.
The toner is charged by a frictional charging action caused by stirring and mixing of the developer, and is electrostatically adsorbed to the electrostatic latent image formed on the photoconductor. Accordingly, proper stirring and mixing are vital to charging the toner adequately. The general structures typically used and the specific operations typically involved in achieving that end are described below.
In many cases, the development device is configured to include a stirring sleeve and a development sleeve. The stirring sleeve supplies the stirred and mixed developer to the development sleeve. The development sleeve supplies the developer to the electrostatic latent image on the photoconductor while magnetically forming upright spikes of the developer on a circumferential surface of the photoconductor. In some cases, each of the sleeves includes a screw auger.
The two sleeves are disposed opposite each other in a helical direction, and are configured to stir the developer while moving and circulating the developer in opposite directions with respect to an axial direction thereof.
As the toner contained in the developer is consumed, the image density is reduced. Thus, the toner needs to be replenished. In the replenishment of the toner, the toner may be poured from an upper portion or an end portion in the axial direction of the stirring sleeve. Usually, the stirring sleeve rotates while mostly buried in the developer. Meanwhile, the replenished toner drops onto the upper surface of the developer in which the stirring sleeve is buried, and thus tends to accumulate on the upper surface of the developer. As a result, adequate stirring and mixing action based on the rotation of the stirring sleeve is difficult to obtain.
In view of the above, various configurations for facilitating the charging of the replenished toner by facilitating the stirring and mixing thereof have been proposed. For example, in a configuration according to one background technique, a toner replenishing tank includes therein a screw member disposed in a longitudinal direction of the tank to rotate on the axis thereof or revolve around another object in the toner prior to the mixing of the toner into the developer, so that the toner is stirred and mixed, and thus charged, by a combination of the rotation and the revolution of the screw member.
Further, in a configuration according to another background technique, an auxiliary stirring member is provided in addition to an existing stirring sleeve to push the developer into spaces formed by a helical blade of the stirring sleeve when the replenished toner is stirred into the developer.
A main purpose of the stirring sleeve using the screw auger is to convey the developer along the helical direction. Therefore, in conveyance by the stirring sleeve, a pushing movement by the helical blade is used to stir the developer. Thus, even if the developer in contact with the helical blade receives the rotational force of the helical blade, the developer is stirred relatively little, since, as noted, the main action of the stirring sleeve is to push the developer in the conveyance direction with the helical blade. Consequently, the stirring sleeve does not sufficiently stir and mix the toner. If the stirring and mixing is insufficient, the frictional charging based on the stirring and mixing becomes unstable. As a result, inappropriately charged toner is scattered to stain and damage surrounding areas, and adheres to a non-image area on the photoconductor to cause scumming.
To prevent a reduction in the amount of charged toner in the developer, it is possible to increase the rotational speed of the screw auger to increase the probability of contact between the toner and the carrier in the developer. However, increasing the probability of contact in this way tends to remove a coating layer of the carrier. As a result, the charging performance of the carrier is degraded, accelerating deterioration of the developer.